It's not just Monday's 216-point selloff: A surge in defensive sectors such as consumer staples and health care suggests a behind-the-scenes shift for the worse in the market.

Investment strategist and money manager Tom Dorsey, president of Dorsey, Wright & Associates, says that one of the most important jobs for an analyst is determining whether to take an offensive or defensive posture. As in football, you have to know which team has the ball.

Right now, some of the technical ratios I follow suggest that the defense is suiting up. Investors should start to think more about keeping profits than chasing...